The present invention relates generally to a method and apparatus for maintaining a vacuum in a rotating member and more particularly relates to a method and apparatus for maintaining a vacuum in a rotating cryostat with a fixed vacuum pump which is operated intermittently.
Typically, a very high degree of reliability is demanded of a turbo-alternator. The same high degree of reliability is generally demanded of a turbo-alternator having superconducting windings. In particular, it is necessary that a vacuum be maintained in a double-walled casing of a cryostat of the turbo-alternator containing a rotor winding over a long period of time. In order to maintain the vacuum, a secondary vacuum pump which requires regular maintenance is typically mounted on board the rotor. The secondary vacuum pump may be utilized to pump the compartment empty when the machine is stopped with the compartment being sealed while the machine is rotating. Alternatively, the secondary vacuum pump has been utilized to effect a continuous pumping of the rotating cryostat from the outside.
Pumping the compartment empty when the machine is stopped is currently being tested in the case of helium reservoirs. In some cases, a sealed compartment does not need to be repumped for several years. In the case of a turbo-alternator, however, the walls of the vacuum compartment are subjected to relatively more severe stresses (centrifugal force, vibrations) and the possibility of cracks in these walls are to be feared. In the case of a turbo-alternator, if the loss of vacuum does not consist of an escape of helium, a gas entering the compartment will condense on the walls of the compartment, since the internal wall of the vacuum compartment is at a very low temperature (cryopumping). On the other hand, if the leakage consists of a loss of helium, the vacuum will be irrevocable broken.
To guard against the loss of vacuum, continuous pumping is sometimes utilized. But continuous pumping is not without danger. In continuous pumping, the vacuum compartment includes a rotating portion, a fixed portion which is connected to the vacuum pump and two rotating seals for connecting the rotating and fixed portions together. A single seal would otherwise be necessary at the end of the machine but that region is already occupied by a helium transfer device. Where a high vacuum is desired, the section to be provided with the vacuum should have a sufficient size. Accordingly, the seals or the systems of seals should have a corresponding large diameter with the peripheral speed of the seals being high. The possibility of a fracture in the sealing at the level of the rotating seals is to be avoided since in the case of continuous pumping the remedy would be almost worse than the ill.
Accordingly, it is an object of the present invention to provide a method and apparatus for maintaining a vacuum in a rotating member using a vacuum pump.
Another object of the present invention is to provide a method and apparatus for maintaining a vacuum in a rotating cryostat using a vacuum pump which is operated intermittently.
Still another object of the present invention is to provide a method and an apparatus for maintaining a vacuum in a rotating cryostat which overcomes the disadvantages of the known methods and devices.
A device according to the present invention enables the vacuum compartment of a rotating cryostat to be connected to a fixed high-vacuum pump and to be responsible for maintaining the vacuum in the compartment. The device includes a fixed portion which is connected to the vacuum pump and a rotating portion that is concentric with the fixed portion. The rotating portion is equipped with a pumping orifice and at least one rotating seal disposed between the fixed portion and the rotating portion. The rotating portion includes at least one valve which rotates with the rotating portion and which closes the pumping orifice of the rotating portion when the vacuum is correct. Means are provided for enabling the valve to be separated from the pumping orifice to bring the vacuum compartment into communication with the pump when the vacuum in the compartment becomes inadequate.